In a product plant for producing finished products such as motor vehicles, necessary parts are supplied by the necessary quantities and at the necessary times to a line side of a production line in accordance with a production schedule of the finished products. FIG. 20 hereof shows the basic idea of a known parts supply system used for replenishing parts to the line side of a production line for producing finished products. The finished products have a higher priority than the parts so that the final products are hereinafter referred to for brevity as “higher products”.
In a diagram shown in FIG. 20 the horizontal axis represents time and the vertical axis represents the inventory quantity of a certain part. The inventory quantity is equal to a number of parts to be replenished to and stocked at the line side of the production line. In FIG. 20, numeral 101 denotes a polygonal line showing variations in the inventory quantity; 102, a parts delivery lead time; 103, an order point (time); and 104, an order quantity. The regular inventory level is set to be 30. The order point is determined by the regular inventory level.
The inventory quantity is kept not to fall below the regular inventory level. To this end, an order point 103 is estimated by calculation based on an estimated amount of use of the parts (slope of the polygonal line 101) and the parts delivery lead time 102 (time taken from the order to the delivery of the parts). When an actual inventory level falls below the inventory level at the estimated order point 103, an order in the desired quantity 104 is issued for replenishment. With this parts replenishment, it is possible to prevent the actual inventory level from falling below the regular inventory level. The order quantity of the parts varies with time. At a time immediately after the delivery of parts has completed, the inventory quantity becomes equal to the sum of the actual inventory quantity and the order quantity and a corresponding quantity of parts remains standing on at the line side of the production line. The inventory quantity should preferably be as small as possible. However, since the actual inventory quantity varies with variations in the part delivery lead time 102 and variations in the quantity of parts used, it is a conventional practice to give a tolerance to the regular inventory level.
FIG. 21 hereof is a graph showing changes in the inventory quantity evidenced when the parts replenishment is executed according to a synchronized delivery system generally called “kanban” system. In FIG. 21, the horizontal axis represents time, and the vertical axis represents inventory quantity. The inventory quantity is divided into two designated by a “first box” and a “second box”, respectively. A regular inventory level, which determines an order point, is set to be 30. Numeral 202 denotes a lead time; 203, a delivery quantity equivalent to an inventory quantity held in the first box. According to the kanban system, control of each type of parts is performed on a parts box basis. In the case of the illustrated kanban system, two boxes of inventory are provided at the line side of the production line. As the assembly of products proceeds, a first box becomes empty whereupon a “kanban” is removed from the empty first box and returned to a relevant parts supplier. The parts supplier then starts to produce a predetermined quantity of parts of the same type as specified in the removed “kanban”. The produced parts are received in a new box and after the “kanban” is attached to the box, the box holding therein the predetermined amount of produced parts is delivered to the product plant.
In this instance, however, if the production of the part at the parts supplier (parts factory) is started after receipt of the “kanban”, the part delivery lead time becomes longer. To deal with this problem, the parts supplier produces the parts in advance and keeps them in stock. The stocked parts are allotted to shipping. The shipping of the stocked parts is treated as a trigger to start producing the parts. The thus triggered production of the parts continues until the produced parts replenish a shortage of inventory caused due to shipment to the production plant.
In this instance if a time required for the production of the parts is shorter than a time that the next order (removed “kanban”) from the product plant takes before its arrival to the parts supplier, the replenishment of the shortage will complete in time. If this is not the case, the parts production process is subdivided so as to provide in-process inventory. The quantity of good-in-progress and the number of “kanbans” used in and between the product plant and the part factory vary greatly depending on the number of “kanbans” used in the parts production line for each unit time.
The “kanban” system operates to tuning or optimization of the respective types of parts by properly setting the number of “kanbans” used and the quantity of goods-in-progress. Thus, once the tuning completes, the “kanban” system does not accept the order quantity per unit time varying widely; if not so the “kanban” system does not operate appropriately. The “kanban” system requires leveling of production schedules used for producing products in the production plant.
FIG. 22 hereof diagrammatically shows another conventional parts production schedule preparing method, wherein in order to ensure that the necessary parts for assembly with finished products can be produced and supplied n accordance with a production order schedule of the finished products, a working time required for production of parts on a part production line and a delivery time required for the delivery of the parts from a part factory to a production plant are calculated, and by calculating back from the product production order schedule, a production schedule of the parts is prepared. In FIG. 22 numeral 401 denotes the production order schedule of products X and Y and numeral 402 denotes a production order schedule of parts Wx and Wy. The parts production order schedule 402 is determined in accordance with the product production order schedule 401. According to the disclosed method, each time the type of products to be produced in accordance with the product production order schedule 401, a setup change should be incorporated in the parts production order schedule 402. Accordingly, if the parts specification varies widely depending on the type of the parts, the number of setup change increases correspondingly, which is time-consuming and may cause a problem that the production of the necessary parts does not complete in time. On the other hand, according to this method, the necessary parts are produced at the necessary times, there are remarkable few pats inventories remaining at the line side of the product production line.
Further examples of conventional parts production scheduling method are disclosed in Japanese Patent Laid-open Publications (JP-A) Nos. 10-156648 and 10-263994. Stated more specifically, JP-A-10-156648 discloses a method of automatically preparing a production schedule with due consideration of the delivery order and time of each type of parts to an assembly line based on a production schedule of the assembly line. On the other hand, JP-A-10-263994 discloses a method of preparing a parts production schedule on the basis of a vehicle body production schedule. According to the disclosed method, based on the premise that parts stock or inventory is provided, an investigation is achieved to determine whether or not the supply of all parts required is in time. When some sorts of parts are found not to be available by the assembly start time, the production schedule is elaborated again so that among those sorts of parts found to be available by the assembly start time, such a sort of parts having an ample amount of time to be used for assembly with the vehicle body is chosen and the production of such sort of parts is postponed.
The conventional parts production scheduling methods discussed above encounter various problems as summarized below.
The conventional scheduling methods are based on the premise that stock or inventory is provided by estimation for each type of parts. This tends to increase a total inventory quantity and requires a large inventory space. Furthermore, the production of a wide variety of parts in a separate manner on the parts production line is low in efficiency. It is therefore highly desirable that the parts be sorted by type to gather or collect parts of the same type in a lot, thereby equalizing or leveling the production load. However, the greater the parts lot size, the longer the lead time before the assembly of the parts with the products begins. This will results in an increased inventory quantity. Conversely, if the parts lot size is reduced, the setup time will increase due to an increased number of die/tool change operation at respective processing stage as well as color change operation in the painting process. In the case where a dedicated program is developed, a problem arises that the dedicated program cannot be readily applied to a process having a different condition. Furthermore, when limiting conditions for its own process are changed, the dedicated program requires a tedious and time-consuming effort for modification thereof.